Corrosion prevention of stainless steel

ABSTRACT

A surface of an article of stainless steel becomes to have increased corrosion resistance by treatment with a heated aqueous hydroxylamine salt solution.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for pre venting an article of stainless steel from corroding.

BACKGROUND OF THE INVENTION

[0002] It is known that a surface of article made of steel can be converted to have increased corrosion resistance by treating the surface with an inorganic or organic corrosion inhibitor. A typical inhibitor is an aqueous phosphate solution. It is further known that the aqueous phosphate corrosion inhibiting solution can contain a hydroxylamine salt so as to form an improved phosphate coating on the treated metal surface.

[0003] U.S. Pat. Nos. 4,149,909, No. 4,865,653, and No. 5,597,465; Published EP Applications No. 0 695 817 A1 and No. 0 315 059 A1; WO 93/03198; and WO 93/20259 teach that a metal such as iron or zinc can be made to have improved corrosion resistance by placing the metal in contact with an aqueous iron phosphate or zinc phosphate solution, respectively, and forming on the metal surface a phosphate coat and that the formation of phosphate coat is enhanced by addition of a small amount of a hydroxylamine salt into the aqueous phosphate solution.

[0004] An article of stainless steel having 11 wt. % or more of chromium element has been known to have high corrosion resistance. Therefore, almost no studies have been made for enhancing corrosion resistance of a stainless steel article by chemical coating, though improvements of the corrosion resistance have been studied with respect to the alloy composition of stainless steel.

[0005] It has been noted by the present inventor that articles made of stainless steel may be corroded when they are placed under very severe conditions such as strong acidic conditions. Otherwise, the articles of stainless steel kept under relatively weak corrosive conditions for a long period of time can be corroded on their surfaces.

[0006] It is known that an aqueous free hydroxylamine solution (which is not in the salt form) can be concentrated from an aqueous diluted free hydroxylamine solution. The concentration is performed by distillation of the aqueous diluted free hydroxylamine solution using a vessel of stainless steel and a distilling apparatus of stainless steel. Since the aqueous free hydroxylamine solution is very corrosive, even the vessel and distilling apparatus of stainless steel is corroded.

[0007] Accordingly, it is an object of the invention to provide a method for effectively preventing an article of stainless steel from corroding.

SUMMARY OF THE INVENTION

[0008] The present invention provides a method for preventing an article of stainless steel from corroding, which comprises keeping the article in contact with a heated aqueous solution of a hydroxylamine salt.

DETAILED DESCRIPTION OF THE INVENTION

[0009] In the method, an article of stainless steel can be preventing from corrosion by keeping the article in contact with a heated aqueous solution of a hydroxylamine salt. The aqueous hydroxylamine salt solution is preferably heated to a temperature of 50° C. or higher, more preferably to a temperature of 60 to 200° C., more specifically to a temperature of 60 to 160° C. If the article of stainless steel is kept into contact with an heated hydroxylamine salt solution in an pressure resistant reaction vessel such as an autoclave, the temperature can be 100° C. or higher. The stainless steel article is kept in contact with the heated hydroxylamine salt solution, preferably for a period of 3 minutes or longer, more preferably 1 to 20 hours.

[0010] The heated hydroxylamine salt solution preferably contains a hydroxylamine salt at a concentration of 0.2 to 50 wt. %.

[0011] Examples of the hydroxylamine salts include hydroxylamine sulfate, hydroxylamine phosphate, and hydroxylamine hydrochloride. Preferred are hydroxylamine sulfate and hydroxylamine phosphate.

[0012] The method of the invention is very advantageous for keeping an article of stainless steel comprising chromium element (especially 15 wt. % or more) ad nickel element in addition to a major portion of iron, from corrosion.

[0013] The present invention is further described by the following examples.

EXAMPLE 1

[0014] In a glass-made flask equipped with a reflux condenser, an aqueous hydroxylamine phosphate solution (concentration: 20 wt. %) was placed. A specimen of stainless steel having the below-mentioned alloy composition was placed in the aqueous hydroxylamine phosphate solution.

[0015] The flask was then placed on an oil bath heated to approx. 100° C., for 8 hours or 24 hours, for performing anti-corrosive treatment.

[0016] (1) SUS 304L Stainless steel (composition: Fe approx. 73%, Ni approx. 9%, Cr approx 18%)

[0017] (2) SUS 316L Stainless steel (composition: Fe approx. 70%, Ni approx. 12%, Cr approx. 16%, Mo approx. 2%)

[0018] (3) SUS 310S Stainless steel (composition: Fe approx. 57%, Ni approx. 19%, Cr approx. 24%)

[0019] Thus treated specimen was weighed, and placed in an aqueous 10 wt. % sulfuric acid solution in a heat-resistant glass bottle. The glass bottle was placed in a thermostat heated to 80° C. and kept for certain periods Then, the glass bottle was taken out from the thermostat. The specimen was recovered from the sulfuric acid solution, washed with water, dried, and weighed.

[0020] Both weights were compared for determining weight loss which was caused by corrosion of the stainless steel specimen.

[0021] For comparison, the equivalent stainless steel specimen having no corrosion-inhibiting treatment was subjected to the abovementioned test for evaluating the resistance in the sulfuric acid solution.

[0022] The results are set forth in the following tables. TABLE 1 (for SUS 304L) Specimen Specimen Specimen Time (hr.) of 8 hrs. of 24 hrs. of no in treatment treatment treatment aq. H₂SO₄ (weight g) (weight g) (weight g) 0 16.4526 17.9558 18.5543 1 — — 18.2378 24 16.4523 17.9557 — 44 16.4521 17.9556 — 68 16.4518 17.9554 — 116 16.4517 17.9553 — 164 16.4513 17.9548 — 188 16.4513 17.9548 — 236 16.4514 17.9554 — 284 16.4516 17.9554 — 380 — 17.9546 — 529 — 17.9544 —

[0023] TABLE 2 (for SUS 316L) Specimen Specimen Time (hr.) of 8 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g) 0 18.2641 18.2103 24 18.1244 17.6733 44 18.0132 —

[0024] TABLE 3 (for SUS 310S) Specimen Specimen Time (hr.) of 8 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g) 0 17.3690 17.5489 1 — 17.4191 8 17.3690 — 23 17.3686 — 47 17.3687 — 71 17.3685 — 95 17.3688 — 119 17.3686 — 161 17.3684 — 209 17.3683 — 281 17.3685 — 329 17.3681 — 455 17.3679 —

EXAMPLE 2

[0025] The procedures of Example 1 were repeated except for replacing the aqueous hydroxylamine phosphate solution with an aqueous hydroxylamine sulfate solution (concentration: 20 wt. %)

[0026] In the corrosion resistance enhancing treatment, a treatment of 72 hours was additionally performed.

[0027] The results are set forth in the following tables. TABLE 4 (for SUS 304L) Specimen Specimen Time (hr.) of 8 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g) 0 15.5943 18.5543 1 — 18.2378 24 15.5944 — 44 15.5940 — 68 15.5937 — 116 15.5935 — 164 15.5931 — 188 15.5931 — 236 15.5935 — 284 15.5937 — 380 15.5930 — 529 15.5929 —

[0028] TABLE 5 (for SUS 316L) Specimen Specimen Time (hr.) of 8 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g)  0 18.2045 18.2103 24 18.0672 17.6733 44 17.9577 —

[0029] TABLE 6-1 (for SUS 310S) Specimen Specimen Time (hr.) of 8 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g) 0 17.7900 17.5489 1 — 17.4191 8 17.7901 — 24 17.7897 — 47 17.7897 — 71 17.7895 — 95 17.7898 — 119 17.7896 — 167 17.7892 — 215 17.7894 — 287 17.7895 — 335 17.7888 — 455 17.7889 — 628 17.7887 —

[0030] TABLE 6-2 (for SUS 310S) Specimen Specimen Time (hr.) of 24 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g) 0 17.9786 17.5489 1 — 17.4191 24 17.9787 — 44 17.9788 — 68 17.9785 — 116 17.9782 — 164 17.9784 — 236 17.9784 — 284 17.9779 — 335 17.9777 —

[0031] TABLE 6-3 (for SUS 310S) Specimen Specimen Time (hr.) of 72 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g) 0 17.5712 17.5489 1 — 17.4191 19 17.5713 — 67 17.5710 — 115 17.5711 — 187 17.5711 — 235 17.5707 — 355 17.5707 — 528 17.5706 —

EXAMPLE 3

[0032] In an autoclave, an aqueous hydroxylamine sulfate solution (concentration: 20 wt. %) was placed. A specimen of stainless steel (3) described in Example 1 was placed in the autoclave.

[0033] The autoclave was then placed on an oil bath heated to approx. 160° C., for 3 hours, for performing anti-corrosive treatment of the specimen.

[0034] The specimen was then placed in the aqueous sulfuric acid solution in the manner described in Example 1, to examine the corrosion resistance of thus treated specimen. The specimen having been subjected to no anti-corrosive treatment was also examined in the same manner.

[0035] The results are set forth in the following Table. TABLE 7 (for SUS 310S) Specimen Specimen Time (hr.) of 3 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g)  0 17.1045 17.5489  1 — 17.4191 124 17.1041 —

EXAMPLE 4

[0036] In an autoclave, an aqueous hydroxylamine sulfate solution (concentration: 5 wt. %) was placed. A specimen of stainless steel (1) or (3) described in Example 1 was placed in the autoclave.

[0037] The autoclave was then placed on an oil bath heated to approx. 160° C., for a hours, for performing anti-corrosive treatment of the specimen.

[0038] The specimen was then placed in the aqueous sulfuric acid solution in the manner described in Example 1, to examine the corrosion resistance of thus treated specimen. The specimen having been subjected to no anti-corrosive treatment was also examined in the same manner.

[0039] The results are set fort in the following Tables. TABLE 8 (for SUS 304L) Specimen Specimen Time (hr.) of 8 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g)  0 17.9588 18.5543  1 — 18.2378 124 17.9583 —

[0040] TABLE 9 for SUS 310S Specimen Specimen Time (hr.) of 8 hrs. of no in treatment treatment aq. H₂SO₄ (weight g) (weight g) 0 35.5074 17.5489 1 — 17.4191 124  35.5066 —

[0041] As is apparent from the results set forth in Tables 1 to 9, the specimens of stainless steel having been treated with an aqueous hydroxylamine salt solution according to the present invention keep their weights even after they are placed in a heated aqueous sulfuric acid solution for several ten hours or several hundred hours. In contrast, the equivalent specimens of stainless steel having been subjected to no anticorrosive treatment loss their weights within one to 24 hrs. In the aqueous sulfuric acid solution, the untreated specimens show blacken surface or give bubble formation within a short period of time. 

What is claimed is:
 1. A method for preventing an article of stainless steel from corroding, which comprises keeping the article in contact with a heated aqueous solution of a hydroxylamine salt.
 2. The method of claim 1 , wherein the aqueous solution is heated to 50° C. or higher.
 3. The method of claim 2 , wherein the aqueous solution is heated to a temperature in the range of 60° C. to 200° C.
 4. The method of claim 1 , wherein the article is kept into contact with the heated solution for a period of 3 minutes or longer.
 5. The method of claim 3 , wherein the article is kept into contact with the heated solution for a period of one hour or longer.
 6. The method of claim 1 , wherein the heated solution contains the hydroxylamine salt in an amount of 0.2 to 50 weight %.
 7. The method of claim 1 , wherein the hydroxylamine salt is hydroxylamine sulfate or hydroxylamine phosphate.
 8. The method of claim 1 , wherein the article is a reaction vessel or a distilling apparatus.
 9. An article of stainless steel having a corrosion resistance surface which is treated with a heated aqueous solution of a hydroxylamine salt. 